Been toying with whether this would work or not, so after some feedback folks.

Make up a container of new salt water, and have the levels exactly the same between the tank and the new water. Then add two tubes which you first fully submerge to remove any air. Then place your thumb over the top of one end and pass them into the other container, and place one end in front of a powerhead to create a positive pressure.

The concept is the positive pressure pushes water slowly from the tank to the bucket, and the other tube will then siphon water back into the tank to maintain the level in the tank. Over time, all the water would mix evenly, then you just chuck out the bucket of water, knowing you have replaced a percentage of the main tank water without having to do much.

I'm thinking of doing this with my coral quarantine tank, which is about 20L, and using a 10L bucket, so that would mean changing about a third of the water using this method.

Always good to envision new concepts, my only fear here would be not enough forward pressure from the power head side pushing water up and over through the pipe. If this method was done with no upward lift required then I see it could be useful.

I'd agree , I think it would need more force to over come the Syphon balance on the outgoing hose . If you've close access to a drain you could add/drill a small overflow from QT just above water level and pipe direct to drain or bucket, it would then just be a case of adding the fresh salt water mix the tank would fill above overflow and would drain away through the overflow . (Excuse my rather quick doodle )

^^ Yeah, it's the pressure I was thinking may not work too. As it pushes up the tube, it'll want to fall back again as it's the tank side that's then sucking back

^ My friend does almost exactly that with his huge freshwater tank. He captures rain water, then puts it through an HMA filter, then it drip feeds his tank. Excess water overflows down the drain.However, this requires emptying my tank, drilling, connectors, etc. The title of the thread is key - I'm too lazy!

How could I generate a little bit of force up the pipe? Airlift is out as it'll probably end up creating a load of microbubbles. Pumps are probably out as it'll be too fast if the water is not heated. I really want a drip feed rate going on.

I was thinking along that lines too but not sure with the unreliability of some of the self priming overflows , a simplified option may be to angle the tank off the level slightly to one end and allow the tank to over flow on one side bit like a Weir or maybe a small cut out for the top of one side out the silicone at the desired level , (I wouldn't recommend this for a bigger tank should anyone else be looking for inspiration ), then sit the tank in a tray with a drain off one end of the tray into a bucket ,